- - Trivial yet extraordinary subject of theory
- - Subject of Einstein's theory
- - Subject of a famous theory
- - "General" topic of a 1916 Einstein book
- - Subject of 15 Across's third 1905 article
- - Subject of 7 Down
- - Theory of 1905.
- - Concern of the physicists.
- - Einstein's theory
- - Subject explored by 19-Across
- - Einstein's famous theory
- - Theory in physics.
- - Famous theory.
- - Historic theory in physics.
- - Scientific theory formulated in 1905.
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